1. Field of the Invention
This invention relates to new and improved amine-terminated piperazine or polyetherdiamine containing polyamide and epoxy resin compositions useful as hot melt adhesives which are curable. In particular it relates to polyamide-epoxy compositions which possess good initial (green) adhesive and cohesive strength properties. It also relates to the cured hot melt adhesives of the invention which possess improved properties, including excellent bond strength, increased water, heat and solvent resistance and increased flexibility and ductility. This invention further relates to the process of manufacture of the hot melt adhesives, both cured and uncured.
2. Brief Description of the Prior Art
It is known that dimer-based polyamides are useful for curing epoxy resins, as in U.S. Pat. Nos. 3,062,773 (Rogier) 2,999,826 (Peerman et al.), 2,930,773 (Renfrew et al.), 2,899,397 (Aelony et al.), 2,890,184 (Foerster), 2,881,194 (Peerman et al.), and 2,707,708 (Wittcoff), however, the polyamides used are not solids at ambient temperatures, but are liquids having a high amine number. The liquid polyamides are mixed with liquid epoxies and allowed to cure. The products possess little cohesive strength until sufficient curing occurs. Certain of the cured products are useful as adhesives, but have shortcomings in that such adhesives are usually rigid and possess limited ductility and flexibility.
It has been known to blend a thermoplastic polyamide with an epoxy resin to produce an adhesive as in U.S. Pat. No. 2,867,592 (Morris et al.). However, the amount of epoxy resin added was limited to a proportion effective to link small numbers of polyamide resin molecules together without providing sufficient cross-linking to cause curing. Thus, such a blend was not a thermoset; it was not capable of being cured to render it infusible.
U.S. Pat. No. 3,488,665 (MacGrandle et al.) teaches a process wherein polyamides are blended with epoxies to provide a product which cures after application to the substrate; however, the product is used to provide a hard, stiffer coating for flexible sheet material. Curable adhesives have been made using polyamide resins and epoxy resins as in U.S. Pat. No. 2,705,223 (Renfrew et al.). But the Renfrew compositions possess inferior properties when applied as adhesives. For example, the Renfrew compositions do not possess good adhesive strength upon cure and provide limited working time after the mixing of the components. In addition, such compositions exhibit poor flexibility, and poor adhesive resistance to heat, water and organic solvents when applied to substrates at ambient temperature.
U.S. Pat. No. 4,082,708 (Mehta) teaches an adhesive system comprising an epoxy resin and a polyamide wherein the polyamide is derived substantially from primary and tertiary amines; specifically, the Mehta polyamides are derived from 1,4-bis-primary amino lower alkyl piperazine having terminal primary amine groups. Although it is suggested that secondary amines can be utilized in making the polyamides as chain extenders and flexibilizers, it is taught that the secondary amines are less desirable reactants and should be buried within the polyamide structure.
The present invention provides an improvement over the prior art in that it provides a two component curable hot melt polyamide with good adhesive properties and with the important added ability to be cured. Thus, it provides hot melt adhesives which have good initial strength properties, but, in addition, which are curable to a state where they have improved adhesive and cohesive strength and gain resistance to remelting (or softening with heat), solvent attack and moisture damage. These characteristics are especially valuable in hot melt adhesives which must perform under challenging conditions of temperature, humidity, and mechanical stress. In addition, the adhesives of the present invention possess greater ductility, flexibility and longer open assembly times. Consequently, the adhesives are easier to employ. The adhesives can be applied to a wide variety of substrates, including substrates at ambient temperature and various types of plastics.